The inquest into the 7/7 London bombings, which killed 52 people on 7 July 2005, cost more than £4.6m, according to official figures.
Expenses included £2.2m for external legal services, £685,581 for IT services, £600,000 for accommodation and £265,262 for staff, while the five-month hearing at the Royal Courts of Justice came to £4,604,721.
The hearing, led by Coroner Lady Justice Hallett, ended in March 2011.
Last May, the court ruled that the emergency services could not have saved any of those who died in the bombings on three Tube trains and a double-decker bus.
But Hallett also raised serious concerns about how MI5 investigates and prioritises suspects, warning that poor record keeping could allow flawed decisions to slip through with "dire consequences".